Customizing Construction and Initialization

Metaclasses can also tap into the __init__ protocol invoked by the type object’s __call__: in general, __new__ creates and returns the class object, and __init__ initializes the already created class. Metaclasses can use both hooks to manage the class at creation time:

class MetaOne(type):
    def __new__(meta, classname, supers, classdict):
        print('In MetaOne.new: ', classname, supers, classdict, sep='\n...')
        return type.__new__(meta, classname, supers, classdict)

    def __init__(Class, classname, supers, classdict):
        print('In MetaOne init:', classname, supers, classdict, sep='\n...')
        print('...init class object:', list(Class.__dict__.keys()))

class Eggs:
    pass

print('making class')
class Spam(Eggs, metaclass=MetaOne):      # Inherits from Eggs, instance of Meta
    data = 1                              # Class data attribute
    def meth(self, arg):                  # Class method attribute
        pass

print('making instance')
X = Spam()
print('data:', X.data)

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In this case, the class initialization method is run after the class construction method, but both run at the end of the class statement before any instances are made:

making class
In MetaOne.new:
...Spam
...(<class '__main__.Eggs'>,)
...{'__module__': '__main__', 'data': 1, 'meth': <function meth at 0x02AAB810>}
In MetaOne init:
...Spam
...(<class '__main__.Eggs'>,)
...{'__module__': '__main__', 'data': 1, 'meth': <function meth at 0x02AAB810>}
...init class object: ['__module__', 'data', 'meth', '__doc__']
making instance
data: 1